This application is based on German Application Nos. DE 198 41 843.4, filed Sep. 12, 1998 and DE 198 43 574.6, filed Sep. 23, 1998, which disclosures are incorporated herein by reference.
The invention relates to a process for carrying out gas-liquid reactions wherein a liquid phase, which represents a reaction component or contains a reaction component in dissolved, emulsified or suspended form, and a gas phase containing a gas to be reacted, are passed through a continuous flow reactor having several reaction chambers in the presence or in the absence of a catalyst. The process relates especially to catalytic hydrogenations and oxidation reactions, such as the production of hydrogen peroxide by the anthraquinone process. The invention further provides a continuous flow reactor having several reaction chambers arranged in parallel for carrying out the process.
Gas-liquid reactions can be carried out in bubble columns of different designs, with or without circulation of the contents of the reactor (cf. Ullmann""s encyclopedia of industrial chemistry, 5th ed., Vol. B4, 276-278 (1992)). The space-time yield (STY) of gas-liquid reactions of the generic type depends to a considerable extent on the flow conditions in the reactor. An increase in the STY can be brought about by intensifying the mass transport processes by means of turbulences in the main flow. In loop reactors this can, to a certain extent, be brought about by increasing the circulation rate. Better bubble distribution can be achieved by reactor inserts, such as, for example, column trays and packings, including static mixer elements. According to another embodiment, the bubble column can also be divided into individual shafts, arranged in parallel. Neither a ratio between chamber width and slit width nor of the slit width forming the basis of the present invention is suggested by this document.
A contact and rectifying column which is also suitable for carrying out catalytic reactions is known from DE-AS 10 67 783. The column comprises several column sections with parallel plates, which form chambers that are open in the direction of the main flow. The plate spacing is 1.2 mm for the 10 m high column given as an example. The document does not teach the selection of a much larger plate spacing.
DE-AS 10 55 501 teaches a column for distillation or other mass transfer purposes. The column contains plate stacks. However, this document does not teach the use of the column for gas-liquid reactions or plate spacing.
GB 620 129 teaches a distillation device which contains plates aligned parallel to the vapour flow instead of packed in a column. Liquid and vapour flow in counter-current.
A flow reactor described in DE 195 36 971 contains substantially parallel capillary flow channels in the direction of the main flow. The width of the channels is 0.08 to 0.4 mm.
A technically significant application of a gas-liquid reaction is the anthraquinone process for the production of hydrogen peroxide (cf. Ullmann""s encyclopedia of industrial chemistry, 5th. ed. Vol. A 13, 447-457 (1989)). In the hydrogenation step of this process, an anthraquinone reaction support dissolved in a working solution is converted to the anthrahydroquinone form with hydrogen in the presence of a suspension catalyst. According to DE patent specification 15 42 089 (corresponding to U.S. Pat. No. 3,423,176) the hydrogenation takes place in a three-phase reaction system in a meandering reaction chamber of consecutive, vertical, alternately narrow and wide pipes. The increase in the rate of hydrogenation was attributed to the increased turbulence achieved in this reactor. However, as taught by U.S. Pat. No. 4,428,923, it was possible to increase the microturbulence, and thus the productivity, corresponding to the space-time yield, in the same process using a meandering tubular reactor consisting of tubes of the same cross section in the rising and falling segments.
The efficiency of the energy transfer (energy dissipation) necessary to produce turbulence is influenced by the turbulence structure. The energy is broken down in cascade fashion via coarse, fine and microturbulences, and the actual chemical processes in a multi-phase system are mainly controlled by microturbulences.
Instead of in a tubular reactor using a suspension catalyst, the hydrogenation step of the anthraquinone process for the production of hydrogen peroxide according to EP-B 0 102 934 can also be carried out using a honeycomb-like catalyst. This catalyst contains many parallel channels, the walls of which are coated with the catalyst. The narrow channels necessarily lead to a considerable pressure drop and, as noted in U.S. Pat. No. 5,071,634, to a weak intermixing of the hydrogen with the working solution, may lead to a phase separation. A disadvantage of catalyst-coated, honeycomb-like elements and catalyst-coated static mixer elements is that they are difficult and expensive to regenerate, for example by exchanging the elements. Another disadvantage is the inadequate transport of heat outwards from the inner areas of the honeycomb-like catalyst support, conventionally made of a ceramic material. The zones for the catalytic reaction and the elimination of heat are thus separated from one another. This makes additional apparatus necessary for eliminating heat.
In the anthraquinone process for H2O2 production according to U.S. Pat. No. 5,071,634, a working solution through which H2 is bubbled is passed through long, narrow static mixer zones with several baffle plates coated with a catalyst. The static mixer zones guarantee good intermixing of the H2 gas bubbles with the working solution and also, owing to the distribution of the flow over the entire cross section, good transport of heat to the cooled external wall, but this version requires greater use of energy owing to the higher pressure drop. In addition, static mixer elements cannot be used if the process is to be operated using a suspension catalyst, because the baffle plates of the static mixer aligned longitudinally and transverse to the direction of flow lead to a catalyst sink (deposit).
In the process of EP-A 0 672 617, a mixture of the working solution and hydrogen is passed at relatively high speed from top to bottom through a fixed bed of catalyst. According to FIG. 2 of this document, the reactor can have vertical plates which consist of two nets with catalyst particles in between. Catalyst and nets fill approximately 30% of the cross section of the reactor, so that a large part of the cross section remains free for the gas-liquid dispersion to flow through. The plates must be permeable so that a constant exchange of the working solution on the left and right of such a plate is possible as a pre-condition of the catalyst reaction. A disadvantage is that, generally, hydrogen is used in excess and therefore must be recycled. Owing to the limited level of catalyst, the reactor volume is higher and thus the use of expensive working solution increases.
The object of the present invention is to provide a process for carrying out gas-liquid reactions which, through the improved design of the reactor, leads to a higher space-time yield (STY) than using known tubular continuous flow reactors. The reactor should have as simple a construction as possible and should be suitable for carrying out gas-liquid reactions in the presence of solids suspended in the liquid medium, such as suspension catalysts. It is another object of the invention to provide an anthraquinone process for producing hydrogen peroxide using a suspension catalyst wherein the STY is increased compared with the process using known, meandering, tubular reactors.
A process for carrying out gas-liquid reactions is provided, wherein a liquid phase, which is a reaction component or contains a reaction component in dissolved, emulsified or suspended form, and a gas phase, containing a gas to be reacted, are passed in co-current, in the presence or absence of a catalyst, through a continuous flow reactor having at least three slit-shaped reactor chambers arranged parallel to one another and designed for the same direction of flow, the ratio of the chamber width b to the slit width s of which is, on average, greater than 3, which is characterized in that a continuous flow reactor is used, the reactor chambers of which have a slit width s in the range of 5 to 100 mm.
Preferred embodiments of the invention relate to the use of the process for the production of hydrogen peroxide by the anthraquinone process, and to particularly preferred designs of the reactor. The ratio b/s of the particularly preferred tubular reactor to be used is usefully in the range of 5 to 100, especially 10 to 50, and the slit width s in the range of 5 to 100 mm, especially 5 to 50 mm.
The continuous flow reactor suitable for carrying out the process comprises a reactor wall in the form of a tube or vessel, an inlet for introducing the reaction components, an outlet for discharging the reacted reaction mixture and at least three slit-shaped reactor chambers arranged parallel to one another and designed for the same direction of flow, the ratio of the chamber width b to the slit width s of which is, on average, greater than 3, and characterized in that the slit width s is in the range of 5 to 100 mm.
In the continuous flow reactors according to the invention, microturbulences are produced by simple means, resulting in acceleration of the reaction. It is known that the mass-related dissipation performance (W/kg) decreases as the macro scale of the large-scale turbulence (=dimension of the largest balls of primary turbulence occurring) increases. As a result of the form of the reactor according to the invention it is possible to minimise the large-scale turbulences and to increase the dissipation performance. From the definition deq=4A/U, wherein deq is the equivalent diameter in a reaction chamber of a different shape corresponding to the hydraulic diameter in a round tube and A corresponds to the area and U the circumference of the other reaction chamber, the equation deq=2bs/(b+s), wherein s is the slit width, corresponding approximately to the macro scale, and b is the chamber width, follows for rectangular cross sections, such as the slit-shaped reactor chambers according to the invention; for s less than  less than b, deq≅2s applies. This equation says that, in the slit-shaped reaction chambers, with the same Reynolds number as in round tubes, only about half the macro scale is present. Thus, in the slit-shaped channels, compared with a tube bundle, the same dissipation is achieved with a much lower pressure drop. Not only the rate of flow, but also wall friction has an effect on the pressure drop. As the laminar area is approached, the pressure drop in the reactor according to the invention is further reduced. It is surprising that these aspects have never yet been applied in the design of continuous flow reactors for gas-liquid reactions.